The present invention relates to the art of footwear construction, more particularly to an improved resilient sole for use in an article of footwear in proximity to the plantar surface of the foot. The sole has a controlled stiffness and exhibits a slow shape recovery following compressive deformation that provides an enhanced stability. The invention also extends to an article of footwear incorporating the improved sole.
Most footwear currently purchased are constructed with soft, highly resilient materials in their soles mainly because users find them more comfortable than stiff soled footwear. In addition, shoes with soft, highly resilient soles are thought to benefit athletic users through xe2x80x9ccushioningxe2x80x9d impacts normally encountered during locomotion or running. Furthermore, these shoes are sometimes thought to provide optimal gait for geriatric users who suffer from frequent falls due to loss of balance.
When these notions are examined scientifically, there is evidence that shoes with soft, highly resilient soles are more comfortable than those with stiffer soles. However, soft resilient soled shoes are not superior in cushioning impact when humans use them. More importantly, recently conducted clinical research clearly suggests that soft highly resilient soled shoes actually destabilize humans of all age groups. In this regard, the reader may refer to the articles entitled xe2x80x9cSHOE SOLE THICKNESS AND HARDNESS INFLUENCE BALANCE IN OLDER MENxe2x80x9d published in 1992, in the Journal of the American Geriatrics Society, and xe2x80x9cATHLETIC FOOTWEAR AFFECT BALANCE IN MENxe2x80x9d, published in 1994, in the British Journal of Sports Medicine, both authored by the present inventor, demonstrating that stability is impaired in humans of all age groups when sole thickness increases and stiffness decreases. The mechanism causing instability appears complex and varies by age. The present inventor has reported in an article published in 1995, in Age and Ageing, entitled xe2x80x9cPROPRIOCEPTION AND STABILITY: FOOT POSITION AWARENESS AS A FUNCTION OF AGE AND FOOTWEARxe2x80x9d, that the mechanism may consist of rapid plantar surface angulation caused by material compression causing loss of foot position awareness. Another possible cause of instability reported by the present inventor in 1988, in the Journal of Testing and Evaluation, entitled xe2x80x9cSENSORY ATTENUATION CAUSED BY MODERN ATHLETIC FOOTWEARxe2x80x9d, consists of sensory insulation caused by the yielding material distributing load more evenly across the plantar surface resulting in a loss of proprioception. The U.S. Pat. No. 4,823,779 issued on Apr. 25, 1989, to the present inventor describes in greater detail the notion of sensory insulation in footwear applications. Another mechanism could be an unstable support base caused by xe2x80x9cbase shiftingxe2x80x9d or tilting of the plantar surface with every gait cycle, or a highly resilient material causing an xe2x80x9cunderdampedxe2x80x9d condition characterized by surface oscillation when load is released rapidly and surface rebound when compressed rapidly, both conditions occurring during locomotion. Oscillations can be observed when load was removed during resiliency testing of a highly resilient material typically used in footwear soles.
In short, footwear design using comparatively stiff and thin soles is a sound approach from the perspective of stability enhancement, but is not satisfactory for most consumers because this design yields less comfortable shoes. Therefore, a clear need exists in the industry to improve the comfort of footwear without resorting to highly resilient sole materials known to impair stability.
An object of the invention is to provide a sole for an article of footwear that offers good stability and yet possesses a good comfort rating.
Another object of the invention is an article of footwear utilizing the aforementioned sole.
The present inventor has made the unexpected discovery that a sole made of a material having a low resiliency offers enhanced stability during locomotion while providing a degree of comfort comparable to what prior art soles made of materials having a highly resilient character.
From a functional point of view, the sole of a shoe can be viewed as a base on which the foot of the wearer rests. In the case of a highly resilient material the recompression activity taking place at every footstep produces a downward movement of the interface plantar surface/sole, causing a transitory xe2x80x9cbase shiftingxe2x80x9d event and perhaps surface oscillation from rebound which may destabilize the wearer. In contrast, a sole made from a low resiliency material offers a much more stable base because the material remains in a compressed condition between footstep without xe2x80x9cbase shiftingxe2x80x9d or rebound. This is referred to an xe2x80x9coverdampedxe2x80x9d condition.
A low resiliency material is characterized by good shape retention properties, xe2x80x9coverdampingxe2x80x9d characterized by reduction of surface oscillation and lack of rebound on compression, and lack of repeated xe2x80x9cbase shiftingxe2x80x9d on recompression. For instance, once the material is subjected to rapid physical deformation there is little or no rebound. Further, when the source of the deformation is removed, it manifests a shape recovery activity as any highly resilient material does, but at a much slower pace, without surface oscillation. In footwear applications this property enables a sole to acquire the shape of the foot for a comparatively long time period, therefore there is no xe2x80x9cbase shiftingxe2x80x9d on recompression because when the compressive effort acting on the sole is temporarily discontinued, such as when the individual raises his foot off the ground during gait, the material of the sole does not have enough time to return to its original configuration. In contrast, a traditional sole made of highly resilient material may rebound when loaded. Further it immediately springs back to its native configuration, perhaps with surface oscillations typical of an xe2x80x9cunderdampedxe2x80x9d system. As a result, when the foot pressure is re-applied during the following footstep the sole is in a vertically expanded condition and produces repeated xe2x80x9cbase shiftsxe2x80x9d, and perhaps rebound and surface oscillation.
The compressed, relatively stiff sole surface encountered by the foot during gait does not create undue discomfort because the sole conforms to the topography of the plantar surface and provides a relatively uniform pressure distribution. In comparison, a flat and substantially unyielding sole creates stress points due to locally acting forces and it is usually perceived by the wearer as being less comfortable.
As embodied and broadly described herein the invention provides a material for use in an article of footwear in proximity to a plantar surface of the foot, said material having a resiliency index in the range from about 0.05 to about 0.5.
The resiliency index is a custom parameter established to quantify the resiliency of the sole following compressive deformation. The test procedure to determine the resiliency index involves observing the amount of shape recovery with relation to time following a predetermined compressive deformation.
More preferably, the resiliency index of the material is in the range from about 0.1 to about 0.35 and most preferably in the range from about 0.1 to about 0.2. In a preferred embodiment, the sole has a hardness in the range from about Shore A2 to about Shore A40. More preferably, the hardness is in the range from about Shore A2 to about Shore A14.
For the purpose of this specification, the term xe2x80x9csolexe2x80x9d is intended to designate all or part of the structures intended to be located in proximity to the plantar surface of the foot, i.e., either in direct contact or at a short distance from the plantar surface. As an example, when the article of footwear is in the form of a shoe, xe2x80x9csolexe2x80x9d designates the material forming the bottom or a layer of the bottom of the shoe such as the sockliner, insole, midsole and the outer sole as well in some specific applications, or a constituent of these parts. In the case of sockliner and insole, it may be removable from the shoe.
The sole does not need to extend under the entire plantar surface. A structure extending only under the ball of the foot or under the heel will be considered a xe2x80x9csolexe2x80x9d.
The article of footwear may be a shoe, boot or sock, among others. In the case of a sock, the sole would normally constitute the bottom part of the foot covering material, in a facing relationship with the plantar surface.
As embodied and broadly described herein, the invention also provides an article of footwear including a sole in proximity of a foot receiving surface of said article of footwear, said sole having a resiliency index in the range from about 0.05 to about 0.5.